Hanoi, Vietnam - Russian President Vladimir Putin said Thursday he "does not rule out" sending weapons to North Korea, where he signed a mutual defense pact this week, while also warning South Korea not to arm Ukraine.

On a trip to Vietnam, Putin said Moscow could send missiles to North Korea, calling it repercussions for the West supplying Ukraine.

"Those who send these [missiles to Ukraine], think that they are not fighting us, but I said, including in Pyongyang, that we then reserve the right to supply weapons to other regions of the world, with regard to our agreements with the DPRK," Putin said.

"I do not rule this one out."

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South Korea on Thursday called the Russia-North Korea treaty a "grave concern," with a senior official saying Seoul would "reconsider" its policy of not sending arms directly to Ukraine.

Putin brushed off the concerns, saying: "South Korea has nothing to worry about."

He warned Seoul not to supply weapons to Kyiv.

"In connections to sending lethal weapons to combat zones in Ukraine, this would be a very big mistake. I hope this will not happen," he said, adding:

"If it will, then we will take the according decision that the current South Korean leadership will probably not like."

South Korea has seen a major growth in international military sales in recent years. But it has a longstanding policy of barring weapons sales into active conflict zones, which it has stuck to despite US and Ukrainian calls to reconsider.